Both the K4 and K5 cases are based on the same general design. The Kindle snaps in place and is held quite securely. In fact, they had to use a pen cap as a prybar to pop one out so I could see behind it. The cases are rather thick, and I'd say it about doubled the thickness of the K4. They also felt sturdy, so in a pinch the SolarKindle case could double for a drop proof case (try at your own risk).
I was particularly interested in these cases because I wanted to see how they were powered. It turns out that they can charge the K4 and K5 off of the 2 metal square on the back. Amazon put them there so the new Kindles can power the case light, but clearly the system can work both ways.
On a side note, this case design implies that a docking station would work for the K4/K5 because the dock would use also those little squares. If any one makes one, let me know. I want one.
The new cases for the Kindle Touch should be coming out in March. They're going to cost the same as the K4 cases, and they should be as widely available. And yes, the K5 is also supposed to get a battery powered lighted case (without the solar panel), just like the one for the K4. That should cost $60, the same as the K4 case.
Now, the last time I wrote about these cases I said that I didn't see the point. $80 wasn't worth the expense for a solar panel to be included in a Kindle case. But I'm going to have to retract that opinion because I hadn't put these cases into their proper perspective.
Amazon's own lighted cases start at $60, the same as SolarFocus' battery powered lighted case. That means that the solar powered ones only cost $20 more and include their own power source. In fact, I like the SolarFocus $60 case more than I like Amazon's. It comes with a battery and can be recharged.
If I wanted a light I would definitely buy this case.